The production of esters by transesterification has for decades been a broadly widespread reaction in the field of acrylics. Nonetheless, processing the crude reaction product and obtaining the desired product in a purified form present numerous technical challenges and have been the subject of numerous developments.
The transesterification reaction involves a “short” chain alkyl acrylate, referred to as being “light”, in the presence of a “long” chain alcohol, referred to as being “heavy”, generally in the presence of catalyst and polymerization inhibitor, according to the following general formula (1):H2C═CHCOOR1+R2OH⇄H2C═CHCOOR2+R1OH  (1)
In order to shift the equilibrium toward the formation of “long” chain alkyl acrylate, it is necessary to eliminate the “light” alcohol produced during the reaction. This reaction is generally accompanied by secondary reactions which produce impurities which must be eliminated with a view to obtaining the desired alkyl acrylate with a high enough level of purity to satisfy the technical requirements associated with its end use as monomer. The alkyl acrylate thus obtained is used in the production of (co)polymers which can be used in numerous fields of application.
Moreover, for obvious economic reasons, the exploitable products present in the crude reaction product mixture, notably unreacted reactants and catalyst, are recycled within the process as far as possible.
Alongside the recycling of exploitable products, the desired product needs to be isolated and purified. For this, numerous separation/purification processes comprising a set of distillations, extractions and/or settling operations are used.
For example, document U.S. Pat. No. 6,977,310 discloses processing the crude reaction product via a first distillation column which separates off the light products and sends the heavy products into a second column to separate the desired product from the catalyst and the polymerization inhibitors.
Document U.S. Pat. No. 7,268,251 discloses various ways of processing the crude reaction product comprising at least four distillation or rectification columns to purify the desired product, including an evaporator for separating off the catalyst.
The process described in document U.S. Pat. No. 7,268,251 has proved to be complicated to implement on an industrial scale due to the optimization of the operating conditions in the four successive distillation/rectification elements to obtain a product of high purity and a satisfactory productivity.
In document JP 2005-239564, use of a distillation column which uses “divided wall” technology (a divided wall column) is proposed to purify a reaction mixture of (meth)acrylic esters. This technology, based on the use of one sole column, nonetheless has drawbacks such as its cost compared to a conventional column (a dividing wall must be installed) and its lack of flexibility with regard to changing the type of streams to be processed.
Also known is document EP 0 968 995 which describes a process for producing alkyl acrylates and which discloses the use of a distillation column, in which the reactants and the desired product are recovered at the top in gaseous form and at the bottom in liquid form, respectively.
In addition, the transesterification reaction in this process occurs directly in the distillation column. The major drawbacks of using a homogeneous catalyst in a distillation column are the dramatic increase in the consumption thereof due to the reflux of the various effluents, and also, in the case of catalyst precipitation, fouling of the distillation column. In the case of a heterogeneous catalyst, the catalyst is located directly in the distillation column and cannot therefore be continuously recycled. Moreover, the efficiency of the catalyst, which decreases over time, directly impacts on the reaction yield. Replacing the catalyst represents a very high cost since the process must be stopped and the column completely cleaned between each catalyst loading.
Processing the crude reaction product derived from the transesterification reaction in the field of acrylics has been the subject of numerous developments. Nonetheless, these processes are still unsatisfactory. There is therefore a great need to be able to have a process for manufacturing alkyl acrylates which is simpler, has better performance, does not have the drawbacks of the prior art on an industrial scale and which meets the requirements of purity of the manufactured product associated with its end use for example as monomer for the manufacture of latex with a low content of volatile organic compounds.
After various experiments, the Applicant has found a process for the production of alkyl acrylate comprising processing the crude reaction mixture by distillation by means of a single simple column, which has never been suggested in the prior art.
According to the teaching of document JP 2005-239564, it is necessary to use a divided wall column to obtain a short chain alkyl (meth)acrylate with sufficient purity, such as butyl methacrylate. The Applicant has found numerous drawbacks to this use, since it is a column with a size generally very much larger than that of a conventional column, therefore with a higher cost, and the use thereof is difficult in terms of optimization of operation, of distribution of the polymerization inhibitors introduced, or of cleaning in the event of deposits on the walls.
Surprisingly, the Applicant has now discovered that using a simple column to purify a reaction mixture of alkyl acrylate in just one step makes it possible to obtain an alkyl acrylate with a purity of greater than 99.8%.
Using a simple column makes use of simplified technology which is more flexible and less costly than that used in document JP 2005-239564. Moreover, it is apparent to the Applicant that this process advantageously applies to the continuous production of heavy alkyl acrylates, that is to say with an alkyl chain comprising more than 4 carbon atoms.
This process makes it possible to both simplify and improve the processing of effluents compared to those of the prior art, to achieve a product of high purity and a high productivity.
Thus, by virtue of using a single simple distillation column for processing the crude reaction product, the process according to the invention makes possible a significant reduction in purification costs compared to those arising from the current processes.